Annually several million people suffer bone fractures caused by accidents or disease in the United States alone, resulting in hospitalizations and a notable economic burden on the U.S. health care system. Moreover, the number of bone fractures caused by an age-related disease, such as osteoporosis, may escalate in industrial nations in the coming years with increasing life expectancy. Many of those fractures are too complex for an external medical treatment and must be surgically fixed using internal orthopedic medical devices such as, for example, implants.
Most orthopedic implants in use are composed of metals. Current metallic implants are made of, for instance, titanium, stainless steel, and cobalt-chromium alloys that will not degrade in the human body after implantation because of their high degradation resistance. Those non-degradable metallic implants have certain drawbacks, including, but not limited to, stress shielding and an increased need for secondary surgeries.
Stress shielding arises after implanting, for instance, plates and screws at the site of a bone fracture. Implants and bone form a composite structure where the stress becomes disproportionally carried. Stiffer components carry larger portions of the load. The materials used in current metallic implants are much stiffer (modulus of elasticity ranging from 100-200 gigapascals) than bone tissues (modulus of elasticity ranging from 10-30 gigapascals). As a result, permanent metallic implants shield the bone from carrying stress. Since bone is an efficient living tissue, it adapts itself to new loading conditions by remodeling and becoming less dense in stress shielded areas. This bone remodeling causes pain in patients with non-degradable metallic implants, especially during the first few years after implantation. Furthermore, the resultant decrease in bone density—called artificial osteoporosis by some orthopedic surgeons—is another side effect of stress shielding that weakens the bone and can lead to refractures. Other negative side effects of stress shielding include, but are not limited to, implant loosening, damage to the healing process and adjacent anatomical structures, osteolysis, and chronic inflammation. To decrease negative effects of stress shielding, many patients with non-degradable metallic implants undergo secondary surgeries to repair, revise, or remove their implants.
Examples of other medical devices that can be used, for instance, to fixate a bone fracture, include, but are not limited to, biodegradable polymer devices, autograft devices, isograft devices, xenograft devices, allograft devices, and ceramic devices. Biodegradable polymer devices have certain drawbacks, including, but not limited to, their relatively low mechanical strength and high rate of wear. An implant having sufficient mechanical strength can withstand the stress of load-bearing applications. Allograft, autograft, and isograft devices are made of human tissue and are biocompatible and biodegradable. Allograft devices have certain drawbacks including, but not limited to, their limited supply. Although ceramic devices can have a relatively high mechanical strength, they also have certain drawbacks including, but not limited to, their brittle and non-biodegradable nature. Through degradation and wear, cracks can easily initiate and further propagate until sudden, catastrophic failure, which damages surrounding tissue.
When a bone fractures, the fragments lose their alignment in the form of displacement or angulation. For the fractured bone to heal without deformity, the bony fragments must be realigned to their normal anatomical position. Orthopedic surgeons may attempt to recreate the normal anatomy of the fractured bone by reduction; that is, an orthopedic surgeon can use an implant as a device that is placed over or within bones to hold a fracture reduction.
The degradation rate of a biodegradable medical device can impact its performance. If degradation rate of a biodegradable medical device is faster than healing rate of a bone fracture, the biodegradable medical device will degrade away and be absorbed by body before the healing process is over. This can cause misaligned fragments and ultimately undesirable deformed bony structure. On the other hand, if the degradation rate of the biodegradable medical device is slower than the healing rate of a bone fracture, the biodegradable medical device will still be in place long after the healing process is over. This can cause stress shielding and artificial osteoporosis. The healing rate of a bone fracture depends on a variety of factors including, but not limited to, physiological conditions, age, weight, height, gender, ethnicity, and overall health, and can differ from one application to the other.
The degradation rates of biodegradable medical devices can be adjusted to approximate the healing rate of surrounding tissues in various applications. For instance, one method for adjusting the degradation rate of a biodegradable medical device is by surface treatment, which can be mechanical or non-mechanical. One example of a mechanical surface treatment is laser shock peening (LSP). LSP uses pressure waves formed by plasma expansion to cause plastic deformation of the implant. Other mechanical surface treatments include, but are not limited to, cutting, grinding, indenting, shot peening, micro-forming, and low-plasticity burnishing.
Because some biodegradable materials (for instance, an alloy of Mg—Ca0.8) are soft and can easily be indented or scratched, several mechanical surface treatments may not be capable of processing a surface without causing permanent damage. For example, shots used in a shot peening technique could easily penetrate into the surface of those biodegradable materials, remain on the surface after the process, and cause contamination after implantation. Contamination that would alter the surface biochemistry could result in several short-term and long-term adverse effects. Machining processes may also produce surface contamination that cannot be removed by normal cleaning. Furthermore, shot peening requires a relatively high amount of cold work, and produces relatively low, shallow, and unstable residual stresses.
One example of a non-mechanical method of adjusting the degradation rate of a medical device includes coating the biodegradable medical device to reduce the degradation rate. Coatings may be formed by several processes including, but not limited to, anodizing, chemical vapor deposition, ion implantation, physical vapor deposition, conversion coatings, plating, immersion, and thermal processes. Ensuring the biocompatibility of a coating material is one drawback. Additionally, coatings may not improve the mechanical strength and fatigue life through improved surface integrity.
Other methods to adjust the surface integrity include bulk modification of the biodegradable medical device including, but not limited to, alloying, forming, hot forming, squeeze casting, deep rolling, equal channel angular pressing, and heat treatments. In forming processes—including, but not limited to, rolling, pressing, extruding, and drawing—the ability the impart a favorable surface integrity can be limited by an implant's geometry. Complex implant geometries can be required to treat some bone fracture and may not always be capable of being processed by traditional forming and casting operations.
Accordingly, there is a need for biodegradable medical devices having appropriate stiffness and mechanical strength to overcome challenges associated with other medical devices, for instance, secondary surgical intervention and stress shielding. There is also a need for biodegradable medical devices having an adjustable rate of degradation, and methods of making the same. The compositions and methods disclosed herein address those and other needs.